(a) What is the law of constant proportions ? Explain with the help of an example. (b) Which postulate of Dalton's atomic theory is in agreement with this law?

(a) In a chemical substance, the elements are always present in definite proportions by mass.
Example: Hydrogen and oxygen combining in the ratio of 1 : 8 to form water.
(b) Relative number of mass atoms which combine to form molecules is constant in a compound.